Suddenly, when I try to save the document I am working on, a box comes up saying: "The document "****" is in the Trash. You can save it to another location.
And when I try to reopen a document I am working on, I get the message that the document is in the Trash.
Any ideas, please?
Thanks,

As a start, run Disk Utility to check/fix your drive. (Disk Warrior is even better and worth the price.) Repair permissions. Try deleting the Nisus plist file in your User preferences folder. You might also redownload NWX from Nisus, as one or two people in the past have reported corrupt installations.